Projecting Global Mean Sea‐Level Change Using CMIP6 Models

• Published in: Geophysical research letters 2021-03, Vol.48 (5), p.n/a
• Main Authors: Hermans, Tim H. J., Gregory, Jonathan M., Palmer, Matthew D., Ringer, Mark A., Katsman, Caroline A., Slangen, Aimée B. A.
• Format: Article
• Language: English
• Subjects: climate sensitivity; global climate models; global mean sea‐level projections; global surface air temperature; global thermal expansion; twenty‐first century
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL092064

The effective climate sensitivity (EffCS) of models in the Coupled Model Intercomparison Project 6 (CMIP6) has increased relative to CMIP5. We explore the implications of this for global mean sea‐level (GMSL) change projections in 2100 for three emissions scenarios. CMIP6 projections of global surface air temperature are substantially higher than in CMIP5, but projections of global mean thermal expansion are not. Using these projections as input to construct projections of GMSL change with IPCC AR5 methods, the 95th percentile of GMSL change at 2100 only increases by 3–7 cm. Projected rates of GMSL rise around 2100 increase more strongly, though, implying more pronounced differences beyond 2100 and greater committed sea‐level rise. Intermodel differences in GMSL projections indicate that EffCS‐based model selection may substantially alter the ensemble projections. GMSL change in 2100 is accurately predicted by time‐integrated temperature change, and thus requires reducing emissions early to be mitigated. Plain Language Summary Climate sensitivity measures how much the Earth's surface warms for a given increase in greenhouse gas concentration. In the new generation of global climate models, climate sensitivity has increased. We explore how the simulations of these models affect global mean sea‐level (GMSL) rise projections in 2100 for three different emissions scenarios. We compute GMSL projections based on simulated global surface warming (which affects land‐ice melt) and thermal expansion of the ocean, using the methods of the fifth Assessment Report of the Intergovernmental Panel on Climate Change. The latest projections of global surface warming are substantially higher than the previous projections, whereas the projections of global thermal expansion are not. Consequently, the upper limits of our GMSL projections increase by 3–7 cm depending on the emissions scenario. This difference will likely become more pronounced beyond 2100. Depending on climate sensitivity, GMSL projections for individual models can differ substantially, implying that using only a subset of models selected based on their climate sensitivity may substantially alter GMSL projections. Since GMSL in 2100 can be predicted well by the cumulative sum of surface warming up to 2100, it is important to reduce the emission of greenhouse gases early to mitigate GMSL rise. Key Points The 95th percentile of total sea‐level change projections in 2100 is up to 7 cm higher for the Coupled Model